1. Field of the Invention
The present invention generally relates to a switching circuit, and more particularly, to a ballast switching circuit.
2. Description of Related Art
Fluorescent lamps are the most popular light sources in our daily lives. To improve the efficiency of fluorescent lamps significantly saves energy. Therefore, in recent development, issues such as efficiency improvement and power saving for a ballast of the fluorescent lamp are deeply concerned. FIG. 1 shows a conventional electronic ballast circuit having a resonant circuit. A half-bridge inverter consists of two switches 10 and 15, which are complementarily switched on/off with 50% duty cycle at a desired switching frequency. The resonant circuit is composed of an inductor 75, a capacitor 70 to operate a fluorescent lamp 50. A capacitor 55 connected in parallel with the fluorescent lamp 50 operates as a start-up circuit. Once the fluorescent lamp 50 starts up, the switching frequency is controlled to produce a required lamp voltage. The drawback of this circuit is high switching loss on switches 10 and 15. The parasitic devices of the fluorescent lamp, such as the equivalent capacitance, vary in response to the temperature variation and the age of the fluorescent lamp 50. Besides, the inductance of the inductor 75 and the capacitance of the capacitor 70 vary during the mass production.
An objective of the present invention is to provide a ballast circuit capable of automatically achieving soft switching operation for reducing the switching loss and improving the efficiency.
Another objective of the present invention is to develop a low-cost ballast circuit with high efficiency performance.